PARSER_BEGIN(Q3)

public class Q3 {

    public static void main(String args[]) throws ParseException {
	Q3 parser = new Q3(System.in);
	try {
	  String c_prog = parser.prog();
	  if (c_prog != null)
	    System.out.println(c_prog);
	} catch (ParseException x) {
	  System.out.println("Exiting.");
	  throw x;
	}
    }

}

PARSER_END(Q3)

/*** Ignoring whitespace ***/
SKIP :
{
    " "
  | "\n"
  | "\r"
  | "\f"
  | "\t"
}

/******************/
/***** TOKENS *****/
/******************/

TOKEN :  /* Keywords  NB - this must be placed before defn of identifiers */
{
  < WHILE : "while" >
| < DO : "do" >
| < BEGIN : "begin" >
| < END : "end" >
| < ASSIGN : ":=" >
}

TOKEN :  
{
  < IDENT : (<LETTER>)+ >
| < #LETTER : ["a" - "z", "A" - "Z"] >
}

TOKEN : /* Punctuation */
{
  < SEMI : ";" >
}


/*************************/
/***** GRAMMAR RULES *****/
/*************************/


/***** Program *****/
String prog() : 
{ String ans=null; }
{
  ans=stat() <EOF>
  { return ans; }
}


/***** Statements *****/
String stat() :
{ 
  String statement=null, expression=null;
  Token id;
}
{
  <BEGIN> statement=statseq() <END>
  { return "{ " + statement + " }"; }
| <WHILE> expression=expr() <DO> statement=stat()
  { return "while (" + expression + ") " + statement; }
| id=<IDENT> <ASSIGN> expression=expr() 
  { return id.image + " = " + expression + ";"; }
}


/***** Sequence of statements *****/
String statseq() :
{
  String statement=null, rest=null;
}
{
  statement=stat() [ <SEMI> rest=statseq() ]
  { if (rest==null) return statement; else return statement + rest; }
}

/***** Expressions *****/
String expr() :
{}
{
   <IDENT> { return new String(token.image); }
}